Crosslinked cellulose products and method for their preparation

ABSTRACT

The invention is a method of making a wet formed, sheeted, readily reslurriable sheeted crosslinked cellulose and the products made by the method. Crosslinked wood pulp fibers tend to be quite brittle. If crosslinked while in sheeted form, the sheets cannot be readily defibered, either in a wet or dry state, without serious fiber degradation. The sheet products of the present invention can be easily redispersed or repulped in water without significant fiber breakage. The present products are made by including within the sheet, while still in wet form, a debonding or softening agent which is preferably added before the latent crosslinking reactant. Most preferably the debonder is added prior to the headbox of a paper machine and the crosslinking reactant is applied near the end of the forming wire or at the press section. The treated sheet is dried conventionally. Crosslinking may occur entirely during drying or during a period of additional heating, usually at a temperature in excess of 100° C. for a short period of time. Conventional debonding agents and crosslinking reactants are suitable. The softening agent apparently reduces or prevents adhesive bonding between adjacent fibers caused by polymer formation external to the fiber under reaction conditions.

This application is a continuation-in-part of application Ser. No.284,885, filed Dec. 15, 1988, which was a continuation-in-part ofapplication Ser. No. 140,922, filed Dec. 28, 1987, which was acontinuation-in part of application Ser. No. 004,729, filed Jan. 20,1987 all now abandoned.

BACKGROUND OF THE INVENTION

The present invention is directed to a method of making a sheetedcrosslinked cellulose and the product resulting from the process. Theinvention is especially directed toward a crosslinked cellulose sheetwhich can later be easily reslurried in water without excessive fiberbreakage.

Crosslinked cellulose products have been described in the chemicalliterature for many years. These products are normally made by reactinga material, usually bifunctional, that will tie together hydroxyl groupson neighboring cellulose chains. Formaldehyde and various derivatives ofurea have been the crosslinking agents which have received the greateststudy. However, many other materials which have actual or latentbifunctional reactive groups have also been reported.

Crosslinked celluloses are of great commercial importance in the textileindustry where they are widely used for the production of wash-and-wearand other wrinkle-resistant types of fabrics. Crosslinked cellulosefluff has also been described for use in disposable absorbent productssuch as diapers. Here advantages is taken of the fact that crosslinkedfibers are normally stiffer than their untreated counterparts. The fluffproducts formed from these fibers are of somewhat lower density (orgreater bulk) and tend to hold retained liquid better under compressiveforces encountered during use of the product.

While the advantages contributed to disposable absorbent products bycrosslinked cellulose fibers are real, products using these crosslinkedfibers have never become commercially important. This is apparentlybecause of the difficulty of making a sheeted crosslinked fiber productthat can be later refiberized at the point of use without creation of anexcessive amount of fines. Unfortunately, crosslinking also results inconsiderable fiber embrittlement. Additionally, most of the crosslinkingagents which have been used serve to give both chemical and physicalbonding between adjacent fibers in the sheets. This, in addition to theincreased fiber brittleness, has made mechanical wet or drydefiberization of sheeted crosslinked pulps impractical. In an effort toovercome this problem, various workers have considered treating sheetedpulp with a latent crosslinking material, fluffing, and then carryingout the crosslinking reaction by heating the cellulose fluff. An exampleof this is seen in Bernardin, U.S. Pat. No. 3,224,926. Van Haaften,Canadian Patent 806,352 treats loose fibers with a crosslinking materialand catalyst. These moist fibers are then expanded into a loose fluffycondition and cured.

The stiffness of crosslinked cellulose fibers can add desirableproperties to certain sheeted pump products. Here it is typical to useonly a portion of crosslinked fibers in the ultimate product. Attemptsto do this have encountered the same problems mentioned earlier. If aproduct is crosslinked in sheeted form, it becomes very difficult toredisperse without serious fiber breakage by normal wet repulpingprocesses employed in paper mills. As noted before, there are twoapparent reasons for this. The strength of a sheeted cellulose productis developed in part by mechanical entanglement of the fibers but, muchmore so, by hydrogen bonding in those areas where fibers overlap are inintimate contact with each other. This hydrogen bonding develops onlywhen the fibers are dry, in a crosslinked sheeted product, when thecrosslinking reaction is normally carried out by heating after the sheethas been fully dried, two phenomena can occur. One of these isinterfiber crosslinking. The reaction occurs in areas of intimatefiber-to-fiber contact and serves to chemically bind the fiberstogether. Perhaps of even greater importance, many of the crosslinkingmaterials that also form thermosetting adhesives under the heatedconditions used in the crosslinking reaction. Scanning electronmicrographs of heated dimethylolurea treated fibers show many smallspherical nodules of ureaformaldehyde resin on the surface and withinthe fiber lumen. These nodules serve to adhesively bond adjacent fibersso that it is very difficult to separate them under any conditionswithout considerable fiber breakage. Because the crosslinked fibers tendto be so brittle, the fibers the themselves will often break leaving thebonded areas between adjacent fibers intact. There is a related sideissue to this phenomenon. It is still an unresolved question as to howmuch of the crosslinking reaction is a surface phenomenon as opposed toan internal one.

Earlier workers in the field have also tried to deal with the problem ofmaking a sheeted cellulose pulp product containing only a portion ofcrosslinked fibers. As one example, Bernardin, in U.S. Pat. No.3,434,918, treats sheeted fiber with a crosslinking agent and catalyst.This is then wet aged to insolublize the crosslinker, so-called "wetfixation." This wet aged fiber is then redispersed before curing. Theredispersed fiber can be mixed with untreated fiber and the mixturesheeted. The final product is then heat cured. In a variation of thisprocess the same inventor, in Canadian Patent 813,616, heat curescrosslinked fibers as a fluff and then mixes this product withconventional papermaking fibers.

These mixtures of crosslinked fibers with untreated fibers arepotentially useful for making products such as filter media, tissues,and towelling where high bulk and good water absorbency are desiredwithout excessive stiffness in the product. Freimark et al, in U.S. Pat.No. 3,755,220, describe making a soft, high wet strength sheet, althoughthis does not use crosslinked fibers. These inventors utilize well knowndebonders or softeners with cationic wet strength resins to gain anincrease in the ratio of wet to dry tensile strength, usually withoutserious loss in absolute values of wet tensile strength. The debonderitself can be cationic or anionic and may be added to the papermakingstock prior to or following the addition of the wet strength resin.

In U.S. Pat. No. 4,204,054, Lesas et al describe spraying unsheeted bulkfibers with a solution of formaldehyde, formic acid and hydrochloricacid. These fibers are then immediately dispersed in a hot air stream atabout 170°-200° C. 1-20 seconds. This appears to give primarily surfacearea crosslinking without serious effect on fiber flexibility. Theinventors note that 10-40% of these fibers can be mixed withconventional fibers to give a sheeted product with good flexibility andwater absorbency.

Unfortunately, the problems encountered handling bulk fibers; i.e.,those in individual loose form as opposed to a sheeted product, havebeen so great as to be commercially nearly insurmountable to the presenttime. The fiber must be dried by flash drying or some similar procedurewhere it is usually suspended in a hot air stream. The dried fiber isthen baled or bagged. Because of the very short fiber length, compactlypackaging a loose fiber form of wood pulp is technically very difficultand expensive. An alternative procedure, where the loose fibrous productmight be prepared at the ultimate point of consumption, has been evenmore unattractive and has met with a wall of resistance by potentialconsumers.

The reader who might be interested in learning more detail of thechemistry of cellulose crosslinking can refer to any of the standardtexts on cellulose. One resource which treats the subject quitethoroughly is by Tersoro and Willard in Cellulose and CelluloseDerivatives, Bikales and Segal, eds., Part V, Wiley-Interscience, NewYork, (1971), pp. 835-875.

Reference was made to use of fiber debonders, also called sheetsofteners in the earlier comments relating to U.S. Pat. No. 3,755,220.These materials can be generally classified as surfactants which areapplied to the fiber while it is still wet, before any hydrogen bondinghas occurred. Most typically they are cationic in nature, based onquaternary ammonium compounds which have one or more fatty substituents.Although not as commonly used, nonionic and anionic types are alsocommercially available, Frequently a combination of a cationic andnonionic type may be employed. These products are widely used within thepulp and paper industry and are commercially available from a number ofsupplies. Similar products are used in the textile industry.

Debonders serve to make a softer sheet by virtue of the fatty portion ofthe molecule which interferes with the normal hydrogen bonding. They arequite commonly used in the manufacture of fluff pulps which will belater converted into absorbent products such as disposable diapers. Theuse of a debonder can reduce the energy required to produce a fluff tohalf or even less than the required for a nontreated pulp. Thisadvantage is not obtained without a price, however, Many debonders tendto reduce water absorbency as a result of hydrophobicity caused by thesame fatty long chain portion which gives the product its effectiveness.In order to overcome this problem, some manufacturers have formedadducts of ethylene or propylene oxide in order to make the productssomewhat more hydrophilic. Those interested in the chemistry ofdebonders will find them widely described in the patent literature. Thefollowing list of U.S. patents provides a fair sampling, although it isnot intended to be exhaustive: Hervey et al., U.S. Pat. Nos. 3,395,708and 3,554,862; Forssblad et al, U.S. Pat. No. 3,677,886; Emanuelsson etal., U.S. Pat. No. 4,144,122; Osborne, III., U.S. Pat. No. 4,351,699;and Hellsten et al., U.S. Pat. No. 4,476,323. All of the aforementionedpatents describe cationic debonders. Laursen, in U.S. Pat. No.4,303,471, describes what might be considered a representativenonanionic debonder.

U.S. Pat. No. 3,844,880 to Meisel, Jr. et al. describes the use ofdeposition aid (generally cationic), an anionic resin emulsion, and asoftening agent which are added sequentially to a pulp furnish toproduce a soft product having high wet and dry tensile strength. Theopposite situation; i.e., low wet tensile strength, is preferred for apulp which is to be later reslurried for some other use.

Croon et al., in U.S. Pat. No. 3,700,549, describe a cellulose fiberproduct crosslinked with a polyhalide, polyepoxide, or epoxyhalide understrongly alkaline conditions. Epichlorohydrin is a preferred material.In their examples Croon et al teach the use of their treated fiber inabsorbent products such as diapers and sanitary napkins. All of thecrosslinking materials are insoluble in water. Croon et al teach threemethods to overcome this problem. The first is the use of vigorousagitation to maintain the crosslinking agent in a fine droplet-sizesuspension. Second is the use of of a polar cosolvent such as acetone ordialkylsulfoxides. Third is the use of a neutral (in terms of being anonreactant) water soluble salt such as magnesium chloride. In avariation of the first method a surfactant may be added to enhance thedispersion of the reactant phase. After reaction the resulting productmust be exhaustively washed to remove the necessary high concentrationof alkali and any unrelated crosslinking materials, salts, or solvents.The method is suitable only for cellulosic products having a relativelyhigh hemicellulose content. A serious deficiency is the need forsubsequent disposal of the toxic materials washed from the reactedproduct. The Croon et al material would also be expected to have allother well known disadvantages incurred with trying to sheet a stiff,brittle crosslinked fiber.

Steiger, in U.S. Pat. No. 3,658,613, teaches a method for "wet"crosslinking a sheeted pulp product so that it can later be defiberizeddry with reduced fiber breakage and knot content. The reaction iscarried out with a crosslinker such as formaldehyde, ureaforms, glyoxaland halohydrins under very strongly acidic conditions; e.g., in areaction medium containing 50% by volume of 37% hydrochloric acid, forabout 30 minutes. The pulp after reaction must be neutralized by a mildalkali and washed prior to drying. The strongly acidic reactionconditions and difficulty of supporting a saturated wet sheet for thenecessary operations over the required period of time would pose almostinsuperable barriers against commercial production.

U.S. Pat. No. 3,819,470, Shaw et al. teach the reaction of a"substantive compound" with cellulose to form a pulp product which canlater be refiberized in water. A debonder may be used at various pointsof addition in the process. The substantive compound is apparently acationic polymer, such as a cationic ureaform wet strength agent. Theinventors clearly teach against the use of crosslinking agents and offera number of reasons why their use would not be suitable in the process.Low retention of the polymer, only about 50%, would seriously affect theeconomics of the process and create a serious waste disposal problem.

Chung, in U.S. Pat. No. 3,440,135, teaches the manufacture of acrosslinked cellulose product which is then later dried in loose fiberform. Sheeted pulp is treated in a bath of a crosslinking agent such asdimethylolurea. A surfactant may optionally be included to improve wetdispersion of the pulp before the drying step. The inventor notes thecritically of wet aging the treated product in the range of 16-48 hoursbefore it is refiberized and dried in a hot air suspension. This agingrequirement would virtually doom the commercial possibilities for theprocess since it would impose on the mill the requirement for keeping alarge inventory of wet sheeted pulp. The additional process expense andspace requirements would make the cost of the product prohibitivelyhigh.

To the knowledge of the present inventor, no one except Chung as notedabove used a debonder with a cellulose pulp which is also treated with acrosslinking agent. One skilled in the art would not expect this to bean effective combination, i.e., they would expect the interfiber bondingpropensities of the crosslinking agents to completely overpower anyadvantage in the reduction of wet or dry strength that might becontributed by the debonding agent. The deficiencies that make the Chungprocess commercially impractical have already been noted.

SUMMARY OF THE INVENTION

The present invention is a method of making wet formed, sheetedcrosslinked cellulose and the products produced thereby which can beeasily reslurried to a free fiber condition without excessive fiberbreakage. The method comprises including within the sheet while stillwet a debonding agent and water soluble or water dispersable latentcellulose crosslinking agent. The sheet thus treated is dried and,during or after drying the crosslinking agent reacts with the cellulose.In the most preferred form of the invention, the debonding agent isadded to an aqueous slurry of cellulose fibers prior to sheet formationand a latent crosslinking agent is added subsequent to sheet formation.This can be readily accomplished by spraying an aqueous solution ordispersion of the crosslinking agent onto the sheet while it still onthe forming wire or in the press section of the paper machine. However,it is within the scope of the invention to add both the debonding agentand the latent crosslinking agent to the wet sheet following sheetformation. In this case it is preferable to add the debonding agent tothe wet sheet prior to the addition of the latent crosslinking agent.The latent crosslinking agent should be added to sheeted cellulose whileit is at a moisture content greater than about 10%, preferably greaterthan about 30%. Immediately after sheet formation the sheet isconventionally dried using standard papermill equipment.

The process of the invention is carried out at a pH generally above 4.It is not necessary to carry out the crosslinking reaction or to handlethe product under highly acidic conditions.

It is within the scope of the invention to use a debonding agent whichmay be either cationic, nonionic or anionic in nature.

The latent crosslinking agent may be selected from any of the followingwell known materials which serve this function. Preferred types areselected from urea derivatives such as methylolated urea, methylolatedcyclic ureas, methylolated lower alkyl substituted cyclic ureas,dihydroxy cyclic ureas, lower alkyl substituted dihydroxy cyclic ureas,methylolated dihydroxy cyclic ureas, and mixtures of any of these types.A presently preferred latent crosslinking materials isdimethyloldihydroxyethyleneurea (DMDHEU,1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone). This material isreadily commercially available in a stable form. Other urea-basedmaterials which are eminently suitable include dimethylol urea (DMU,Bis[N-hydroxymethyl]urea), dihydroxethyleneurea (DHEU,4,5-dihydroxy-2-imidazolidinone), dimethylolethylene area (DMEU,1,3-dihydroxymethyl-2-imidazolidinone), and4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone (DDI,dimethyldihydroxyethyleneurea).

In addition to those latent crosslinking agents based on urea, othermaterials that are suitable are polycarboxylic organic acids. Amongthese 1,2,3,4-butanetetracarboxylic acid is a presently preferredmaterial.

All of the crosslinking agents just described may be reacted with thecellulose either during normal drying of the sheeted material orsubsequent to this time by raising the dried sheet to an elevatedtemperature, preferably above 100° C.

A neutral or acidic catalyst may be included with the latentcrosslinking agent to increase the reaction rate between the crosslinkerand the cellulose. Acidic salts are particularly useful as catalystswhen the urea-based materials are employed. These salts may typically beammonium chloride or sulfate, aluminum chloride, magnesium chloride ormixtures of these or many other similar materials. Alkali metal salts ofphosphorous-containing acids, such as sodium hexametaphosphate andsodium hypophosphite, with or without additional oxalic acid, are usefulcatalysts for 1,2,3,4-butane carboxylic acid.

The crosslinking agent is typically present in an amount in the range of2-200 kg/t, preferably 20-100 kg/t, of cellulose fiber. Similarly, thedebonding agent is generally present in an amount of about 0.1-20 kg/t,preferably 1-10 kg/t, of cellulose fiber.

A particular advantage of the new process is found in the lack of anyneed for washing the sheeted crosslinked product after the crosslinkingreaction is completed.

It is an object of the present invention to provide a sheetedcrosslinked cellulose product which can be readily reslurried in waterto a free fiber condition without excessive fiber breakage or energyinput.

It is a further object of the invention to provide a method ofmanufacturing such a product.

It is another object to provide a method and product as described whichcan be conveniently and readily made on conventional papermakingequipment.

It is yet another object to provide a product of the types describedwhich can be readily redispersed in water and mixed with other types offibers, which mixtures can be resheeted to give products having noveland useful properties.

It is an additional object to provide a sheeted crosslinked cellulosepaper product that can be manufactured on standard unmodifiedpapermaking equipment in a wholly conventional manner.

It is also an object to manufacture a crosslinled cellulose productunder neutral or acidic conditions in which the pH is not below about 4.

These and many other objects will become readily apparent to thoseskilled in the art upon reading the following detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The sheeted crosslinked cellulose products of the present invention areintended for use as manufactured, or for remanufacture by a process thatinvolves redispersing the product in water, usually for admixture withother fibers, followed by resheeting. It is not a primary goal orintention of the invention to produce sheeted products useful inprocesses that involve dry mechanical defibering, even though somespecies may perform satisfactorily under these conditions. The presentinvention provides a sheeted crosslinked cellulosic product thatcontributes good bulk and absorbency to a remanufactured sheet withlittle or not loss of fiber integrity or length during theremanufacturing process.

While the individual use of debonding agents and crosslinking reagentshave been both known for some time in the pulp and paper industry, thesehave never before been used in combination in a sheeted pulp product, tothe knowledge of the present inventor. It was totally unexpected thatthe debonders would continue to function as such that treatment of andreaction of the fibers with crosslinking materials. This is especiallythe case since many crosslinking agents will, at least to some extent,form polymers as a side reaction while reacting with the cellulose. Inmany cases these polymeric side reaction products serve as powerfuladhesive materials. As one example, the efficiency of urea-basedpolymers as bonding agents for cellulosic materials is well known. Manyof the precursors of these urea adhesives are the identical materialsthat are also highly effective cellulose crosslinking agents.

To the inventors best knowledge, any latent cellulose crosslinkingcomposition is effective in the present invention. Those that can bereacted at relatively low temperatures in short periods of time duringor after normal drying are preferred from a technical and economicstandpoint. The urea-based crosslinking materials seem to fill thisrequirement well since their reaction speed can be greatly acceleratedwith small amounts of inexpensive acidic salt catalysts. Other classesof crosslinking agents can probably be similarly accelerated as well. Norepresentation is made here that any of the processes described in thefollowing examples have been optimized.

In similar manner, it appears that any class of debonding agent will besatisfactory, although there is some indication that cationic types maybe superior to nonionic or anionic materials. Again, the systemsreported here have not been optimized.

Cationic debonders are most usually based on quaternary ammonium saltshaving one or two lower alkyl substituents and one or two substituentsthat are or contain a fatty, relatively long chain hydrocarbon. Most ofthese fall into one of four general types as follows: ##STR1## where R₁and R₂ are methyl, ethyl, or hydroxyethyl, R₃ is a hydrocarbon having1-40 carbon atoms, R₄ is a hydrocarbon having 10-40 carbon atoms, E isan oxyalkylene group having 2 or 3 carbon atoms, m is an integer from1-20, n is an integer from 0-20, and X is Cl or SO₄, said hydrocarbonsubstituents being selected from linear and branched alkyl or alkenylgroups, and branched and linear alkyl and alkenyl substituted phenylgroups. Most typically R₃ will have from 1-22 carbon atoms and R₄ from10-22.

Originally most debonders were Type 1 fattyalkyl di- or trimethylammonium compounds. These have now been superceded in many cases by theother types since they may induce an undesirable hydrophobicity.

The Type 2 debonders, diamidoamine types, are quite inexpensive and arewidely used as fabric softeners.

Dialkyl alkoxylated quaternary ammonium compounds (Type 3) are widelyused in making fluff pulps for disposable diapers since the polyethyleneor propylene oxide chains give better hydrophylicity and cause lessdegradation of absorbency, especially when compared with Type 1compositions.

The imidazoline materials that comprise Type 4 materials are somewhatnewer materials. However, they are also now widely used.

Nonionic materials that can serve as debonders comprise a very largeclass of materials. Principal among them are adduct type reactionproducts of fatty aliphatic alcohols, fatty alkyl phenols and fattyaromatic and aliphatic acids with ethylene oxide, propylene oxide ormixtures of these two materials. Most typically the fatty portion is ahydrocarbon chain having at least 8, more typically 10-22, carbon atoms.Other useful nonionic debonders include partial fatty acid esters ofpolyvalent alcohols and their anhydrides wherein the alcohol oranhydride has 2-8 carbon atoms.

Anionic debonders also include a large class of materials, includingmany having surfactant properties. In general these are sulfated fats,fatty esters, or fatty alcohols. They also include fatty alkylsubstituted aromatic sulfonic acids. The fatty substituent groups mayhave from 8-40 carbon atoms, more typically from 10-22 carbon atoms.

In the most preferred practice of the invention the debonder will beadded to the cellulose fiber stock at some point before the headbox ofthe paper machine. When anionic or nonionic debonders are used it isnormal practice to also use a cationic retention aid at the point of orimmediately prior to their addition. Otherwise, they will have very poorsubstantivity to the cellulose fibers. It is within the scope of theinvention to add both the softener and latent crosslinking agent afterformation of the sheet. In this case it is not always necessary to useretention aids with nonionic or anionic debonders.

The following examples will illustrate the best modes presently known tothe inventor for carrying out the present process and making theresulting products.

EXAMPLE 1

The following procedure was used to make laboratory handsheets forevaluation. A 25 g (dry weight) sample of unrefined cellulose pulp wasreslurried in a Waring Blendor at about 2% consistency for 20 seconds.After 5 seconds of agitation, one of the commercially availablesoftening agents was added to the blender in amounts ranging from 0%(for control samples) to 2% based on dry pulp. Most typical usage wasabout 0.5% (5 kg/t), on an as received basis. The reslurried, softenertreated pulp was further diluted to a volume of about 6800 mL withwater. This slurry was formed into a sheet on a standard 8×8inch(203×203 mm) Noble and Wood laboratory sheet mold, using a 150 meshstainless steel screen. The sheet was removed from the former andpressed between synthetic fiber felts so that the moisture content wasreduced to about 50%.

The moist sheet prepared as above was then immersed into a bathcontaining a known concentration of a latent crosslinking agent andcatalyst, if the latter component was used. After immersion the sheetpicked up sufficient treating liquid so that its consistency was reducedto about 13.5%. If was again pressed between felts to about 50% fibercontent. It can be readily calculated to show that the final pickup oflatent crosslinking agent and catalyst, based on pulp, was about 85% ofthe concentration in the bath. The handsheet was then drum dried toabout 5% moisture content.

Depending on the particular crosslinking agent and/or catalyst used, thecrosslinking reaction with the cellulose occurred either during thedrying step or in an oven curing stage following drying.

EXAMPLE 2

The bulk density of a crosslinked pulp sheet is dependent on a number ofinteracting factors; the physical nature of the cellulose, the type andamount of softener used, the type and amount of crosslinking agentand/or catalyst used, and the time and temperature of the crosslinkingreaction. The effect of time-temperature relationship for one set ofconditions can be seen in the following example using laboratoryhandsheet samples.

A bleached Douglas-fir kraft pulp was reslurried as described in Example1 and treated with 0.5% as received of Berocell 584 softener. Thismaterial is a quaternary ammonium based softener believed to beprincipally a fatty substituted oxyalkylatedphenol dialkyl quaternaryammonium chloride (see the Type 3 quaternary formula noted earlier).This is compounded using 30% of the quaternary compound with 70% of anonionic polyoxyalkylene composition. It is available from BerolChemical Co., Reserve, La. After sheeting and pressing, the handsheetswere treated with a 10%, as received basis, aqueous solution of Arotex900 latent crosslinking agent. Arotex is a registered trademark of andis available from American Cyanamid Company, Wayne, N.J. It is believedto be a dimethyloldihydroxyethyleneurea product and is sold as anaqueous solution at about 45% solids concentration. For every 100 partsof the Arotex 900 solution, 30 parts by weight of Arotex Accelerator 9catalyst solution were used. This is a 30% by weight solution of acidicsalts believed to be aluminum and magnesium chlorides. Retention of thelatent crosslinking agent, on a 100% solids basis, was calculated to be3.78% of the dry cellulose present. The dried sheets were cured at 150°C. for 3 minutes.

In order to determine the reslurring and bulking properties of thetreated fiber a 3.5 g, dry weight, sample was torn into small pieces andreslurried in about 2 L of water in a British Disintegrator. Agitationwas continued until the slurry was smooth and free of obvious knots orfiber bundles. The number of revolutions to this point was counted andis an indicator of the ease with which the material can be redispersed.The slurry was then sheeted in a standard 61/4 in (159 mm) TAPPI sheetmold. After draining it was vacuum couched but was then drum driedwithout pressing. Bulk density was measured on the dried samples. Highbulk values are generally as indication of high fiber stiffness.However, high bulk values cannot be obtained if there has been anysignificant amount of fiber breakage during reslurring. For this reason,bulk density is also strongly indicative of fiber length and of anyfiber damage during reslurrying.

                  TABLE I                                                         ______________________________________                                                        Control                                                                              Crosslinked Pulp                                                       (Un-   No De-    De-                                                          treated)                                                                             bonder    bonded                                       ______________________________________                                        Disintegration Energy, revs.                                                                    15,000   125,000   20,000                                   Handsheet Bulk Density, cm.sup.3 /g                                                             3.1      9.5       16.5                                     ______________________________________                                    

EXAMPLE 3

The reaction conditions; i.e., time, temperature, and catalystconcentration, between the potential crosslinking agent and thecellulose affect the bulking potential and ease of reslurrying of thesheeted product. A series of handsheets was made according to theprocedures outlined in Examples 1 and 2. However, this time the amountof as received Arotex 900 in the treatment bath was varied in 5% stepsbetween 0% and 20%, resulting in pickups by the fiber varying between1.9 and 7.6%, as calculated on a dry materials basis. A constant weightratio of 10:3 between as received crosslinker and catalyst wasmaintained for all samples. This ratio may be expressed as 5:1 on a drysolids basis. The resulting 203×203 mm Noble and Wood handsheets wereresheeted as in Example 2 in the TAPPI sheet mold to obtained samplesfor bulk densities. Results were as follows:

                                      TABLE II                                    __________________________________________________________________________    Effect of Curing Conditions on Handsheet Bulk Density                         TAPPI Handsheet Bulk Densities, cm.sup.3 /g                                            Reaction Temperature                                                 Crosslinker Solids                                                                     120° C.                                                                            140° C.                                                                            160° C.                               Based on Pulp, %                                                                       1 min                                                                             3 min                                                                             5 min                                                                             1 min                                                                             3 min                                                                             5 min                                                                             1 min                                                                             3 min                                                                             5 min                                __________________________________________________________________________    0                        3.2                                                  1.89     3.8 6.7 5.2 7.6 9.5 10.5                                                                              10.3                                                                              14.1                                                                              15.8                                 3.78     9.7 13.2                                                                              16.0                                                                              15.3                                                                              15.7                                                                              17.7                                                                              19.3                                                                              14.6                                                                              14.5                                 5.67     6.7 16.8                                                                              20.4                                                                              18.6                                                                              17.7                                                                              16.7                                                                              20.0                                                                              14.8                                                                              14.8                                 7.56     9.0 19.3                                                                              19.7                                                                              19.6                                                                              16.1                                                                              17.4                                                                              20.0                                                                              --  --                                   __________________________________________________________________________

It is readily apparent that with the present crosslinker system, TAPPIsheet bulk density increased directly with increases in crosslinkerusage, reaction time, and reaction temperature. However, little changewas seen in sheet bulk with increase in reaction time from 3 to 6minutes, especially at the two higher curing temperatures. Likewise,there does not appear to great advantage at reacting at the highertemperature of 160° C. compared with 140° C. In fact, at highercrosslinker usages the higher temperature may cause undesirable fiberembrittlement.

EXAMPLE 4

A series of samples was made using a 10% Arotex 900 bath treatment andcomparing the Berol 584 softener, used in Examples 2 and 3, with anonionic softener and a nonionic/cationic softener combination. Thenonionic material was Triton X-100, a nonylphenol type. Triton is aregistered trademark of and the product is available from Rohm and HaasCo., Philadelphia, Pa. The samples without softener and with thecationic softener were made as in Example 2. In the case where thenonionic softener by itself was used in combination with thecrosslinking agent, both were included in the crosslinker bath and nosoftener was added prior to sheet formation. Estimated concentration ofnonionic material solids incorporated into the final product, based ondry cellulose, is 0.8%. When the cationic/nonionic combination was used,the cationic was added as in Example 2, prior to sheeting, and thenonionic was included with the crosslinking agent as just described. Inaddition to sheet bulk density values, disintegration energy wasestimated by noting the number of British Disintegrator revolutionsnecessary to give a uniform fiber dispersion without nots or fiberclumps. Results were obtained as shown in Table III.

                  TABLE III                                                       ______________________________________                                                       Disintegration                                                                            Handsheet Bulk                                     Sample         Energy, revs                                                                              Density, cm.sup.3 /g                               ______________________________________                                        No debonder    120,000     11.9                                               Cationic debonder                                                                            22,500      24.7                                               Nonionic debonder                                                                            62,500      16.9                                               Cationic/nonionic                                                                            62,500      18.4                                               ______________________________________                                    

The nonionic softener significantly improves ease of dispersibility andincreases bulk value. However, it is not as effective here as thecationic debonder and, when used in combination under these conditions,reduces the effectiveness of the cationic material.

EXAMPLE 5

A major use of the products of the invention is expected to be infiltration medium. Here some portion of the crosslinked fiber wouldnormally be repulped, blended with untreated fiber, and resheeted. Amajor contribution of the crosslinked fiber is porosity control and, insome cases, it can make higher porosities possible than can now bereadily attained. One common measure of the expected behavior of afilter medium is air porosity. A number of test procedures are employed.The particular one chosen is in part dependent on the expected airresistance of the sheet. The tests on the present product were conductedon sheets having a basis weight of 160+5 g/m² by measuring the pressuredrop caused by an air flow of 0.085 m³ /min.

Sheets were formed using 3.5 g, dry weight, of pulp dispersed in aBritish Disintegrator in about 2 L of water until a uniform slurry wasproduced. Sheets were formed in a standard laboratory British SheetMold, couched at 68.9 kPa, drum dried between blotters, and heated for1.5 minutes at 150° C. to react the cellulose and crosslinker. Beforetesting sheets were conditions to equilibrium at 50% RH at 23° C.

For the tests reported below in Table IV, Arotex 900 was used in bathconcentrations of 1, 3, 5, 10, and 15% and the fiber was treated beforesheeting with 0.5% berocell 584 debonding agent.

                  TABLE IV                                                        ______________________________________                                        Bulk and Air Resistance of Crosslinked Fiber                                  Crosslinker Solids                                                                         Handsheet Bulk                                                                            Air Resistance                                       Based on Pulp, %                                                                           Density, cm.sup.3 /g                                                                      Pressure Drop, mm                                    ______________________________________                                        0            3.7         37                                                   0.38         --          19                                                   1.13         8           4.3                                                  1.89         10          3.0                                                  3.78         21          0.5                                                  5.67         23          0.5                                                  Untreated Control.sup.(1)                                                                  5.5         3.3                                                  ______________________________________                                         .sup.(1) A commercially available prehydrolyzed, cold caustic extracted       southern pine kraft pulp widely used in filter media.                    

The desirable air resistance properties contributed by the readilyredispersible crosslinked cellulose pulp are immediately apparent.

EXAMPLE 6

Another expected major use of the products of the present invention isin tissues and toweling in order to maintain high bulk and softness withgood water absorbency. To show the effectiveness of the crosslinkedmaterial, a sample was prepared as in Example 3 using a bathconcentration of 15% Arotex 900. This resulted in a pickup ofcrosslinker solids based on dry pulp of about 5.7%. Varying amounts ofthis product were reslurried and added to fiber obtained by reslurryingtwo popular brands of toilet tissue. One of these, Tissue A, was aconventional hot drum dried product while the other, Tissue B, wasoriginally dried using heated air passed through the tissue to maintainsoftness.

Sheets were formed in a standard laboratory British Sheet Mold asdescribed in the previous example using 0.44 g, dry weight, of fiber togive a final sheet having a basis weight of about 24 g/m2. In additionto the bulk density value, softening efficiency of the crosslinked pulpin the ultimate sheet was estimated. This was calculated by taking theratio (% increase in bulk density over a control sample) divided by (%treated pulp using the sample). Results are given in the followingtable:

                  TABLE V                                                         ______________________________________                                        Addition of Crosslinked Pulp into Tissue Furnish                                        Tissue A     Tissue B                                               Treated Pulp Used                                                                         Bulk,              Bulk,                                          in Furnish, %                                                                             cm.sup.3 /g                                                                           Efficiency cm.sup.3 /g                                                                         Efficiency                               ______________________________________                                         0          4.0     --         3.5   --                                       10          4.5     1.2        4.9   3.7                                      20          5.4     1.7        5.8   3.1                                      40          7.7     2.3        8.4   3.0                                      60          11.2    3.0        13.0  4.5                                      ______________________________________                                    

The effectiveness of the crosslinked pulp at increasing bulk isimmediately apparent. It was unexpected that the bulking efficiencywould increase as higher levels of crosslinked pulp were used.

EXAMPLE 7

In order to compare different cyclic urea compositions a supply ofdihydroxyethyleneurea (DHEU) was prepared by reacting equimolar portionsof glyoxal and urea, generally as taught in British Patent 717,287. Thiswas compared with the Arotex 900 dimethyloldihydroxyethyleneurea(DMDHEU) used in the previous examples. Using 15% of each compound isrespective treatment baths, samples were made up as described in Example2. 30% of Arotex Accelerator 9 was used with the Arotex 900 in thetreatment bath while 30% of a 10 g/L zinc nitrate solution was used withthe DHEU. After drying, reaction times between the crosslinking agentand cellulose of 1-3 minutes were used at a temperature of 140° C. TableVI shows that nearly identical bulk values were obtained with the twocompounds.

                  TABLE VI                                                        ______________________________________                                                     Handsheet Bulk Density cm.sup.3 /g                               Reaction Time, min                                                                           DMDHEU        DHEU                                             ______________________________________                                        1              25            24                                               3              28            29                                               5              26            25                                               ______________________________________                                    

The two compounds appear to be about equally effective and there appearsto be no advantage for using longer reaction times.

EXAMPLE 8

The following tests were made to show the effectiveness of other genericclasses of chemical crosslinking agents for cellulose.

A 20 g (oven dried weight) sample of never dried Northwest bleachedkraft softwood pulp at 35% consistency was weighed out and placed in aBritish Disintegrator, made up to 2 L with deionized water, and agitatedfor 5 min at 600 rpm. The reslurried fiber was then dumped into a 8"×8"(203×203 mm) Noble and Wood laboratory sheet mold containing 4 L ofdeionized water. More water was added up to 2" below the top of the moldto give a total of about 6.3 L. A perforated stainless steel platesomewhat less than the cross-sectional size of the sheet mold, with a12" handle, was inserted into the sheet mold and moved up and down threetimes in rapid succession and 1 time slowly. The valve on the bottom ofthe sheet mold was opened and the stock drained through the screen. Thepad of pulp remaining of the screen was removed, placed betweensynthetic fiber felts, and squeezed very gently through press rolls. Thefinal weight of the pad was 65 g (45 g water and 20 g pulp).

A 1% solution of as received Berocell 541 (Berol Chemical Company,Reserve, La.) was made up and sprayed onto both sides of the pulp pad(approximately equal distribution) to obtain an uptake of 1% softenerbased on OD pulp. After 3 min a 15% solution of maleic anhydride (MA) inwater was sprayed onto the pulp pad in the same manner for a 15% (basedon OD pulp) material uptake. The pad was then placed between 2 dry 8"×8"pulp blotters and fed through the drum dryer until the pad wascompletely dry. It was then transferred to a watch glass and placed in a160° C. oven for 15 minutes. A 3.5 g sample was torn off the pad andreslurried in the British Disintegrator (using 2 L of deionized water)for 5 min at 600 rpm. The slurry was passed into a 61/4" TAPPI sheetmold and processed to a hand sheet. The pad was drum dried withoutpressing, conditioned at 50% RH and 23° C., and measured for bulkdensity.

In the like manner, additional samples were treated with1,2,3,4-butanetetracarboxylic acid (BTCA),4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone (DDI), with and withoutsoftener. The samples made with DDI included 1% (based on pulp) of amixed AlCl₃.MgCl₂ catalyst. All samples were run in duplicate. Resultsare given in Table VII. Sheet formation was graded relatively asfollows:

1-uniform good formation

2-fairly good formation without nits (undispersed fiber clumps)

3-fair formation with some knots or flocs present

4-very poor formation with original sample not completely redispersed.

                  TABLE VII                                                       ______________________________________                                        Bulk Values Using Various Cellulose Crosslinking Agents                                        Bulk Value                                                                              Relative                                           Treatment        cm.sup.3 /g                                                                             Dispersibility                                     ______________________________________                                        Untreated        4.90      2                                                  1% softener      4.87      1                                                  15% MA           5.38      3                                                  15% MA + 1% Softener                                                                           6.51      1                                                  15% DDI          5.82      2                                                  15% DDI + 1% Softener                                                                          7.73      1                                                  15% BTCA         6.3.sup.(1)                                                                             4                                                  15% BTCA + 1% Softener                                                                         10.17     1                                                  ______________________________________                                         .sup.(1) Best estimate attainable due to very poor formation             

In all cases, except with glyoxal, the bulk value was improved when asoftener was incorporated into the cellulose prior to addition andreaction of the crosslinking agent. Tests made under the otherconditions have shown glyoxal to be an effective material in theapplication. All of the softened samples reslurried more readily thanthose without the softener.

EXAMPLE 9

The work described in Example 2 was repeated in order to make a fiberlength measurement study on reslurried sheets. One difference this timewas an increase in the concentration of Arotex 900 from 10% to 15% on anas received basis in the treatment bath. A second difference was the useof 0.5% Varisoft 727 as the debonding material. Varisoft is a registeredtrademark of Sherex Chemical Company, Dublin, Ohio. Verisoft 727 is aformulated alkyl diamidoamine type quaternary compound in which thealkyl substituents are typically oleyl or tallow based. The compositioncontains about 30% quaternary material.

The higher concentration used here, as compared with Example 2, would beexpected to increase the ultimate concentration of the latentcrosslinking material in the cellulose fiber from about 3.8% to 5.7% andalso to increase the brittleness of the crosslinked fibers. Fiber lengthdeterminations were made using a Kajanni Type FS-100 automatic fiberlength analyzer, available from Kajanni Electronics Co., Kajanni,Finland. As before, the samples were dispersed in the BritishDisintegrator until smooth, knot free slurries were attained. Resultsare given below.

                  TABLE VIII                                                      ______________________________________                                                         Control                                                                              Crosslinked pulp                                                       (Un-   No De-   De-                                                           treated)                                                                             bonder   bonded                                       ______________________________________                                        Disintegration Energy, revs                                                                      15,000   138,000  25,000                                   Handsheet Bulk Density, cm.sup.3 /g                                                              3.1      7.9      20.9                                     Weighted Ave. Fiber Length, mm                                                                   3.0      1.3      2.4                                      ______________________________________                                    

The debonded crosslinked pulp retained 80% of the fiber length of thecontrol sample with very little more disintegration energy beingrequired to redisperse the sheets in water. The crosslinked sampleswithout debonder had only 43% of the average fiber length of the controlsamples. This major reduction is probably due to the very much higherenergy required to obtain a smooth, knot-free fiber slurry.

EXAMPLE 10

A set of experiments was made to show the relative effectiveness ofother types of quaternary debonding agents when used in conjunction withthe Arotex 900 dimethylodihydroxyethyleneurea (DMDHEU) latentcrosslinking agent. The type numbers listed below refer to those notedearlier in the description of preferred embodiments.

Variquat and Adogen are registered trademarks of Sherex ChemicalCompany. Variquat 638 is described as a methyl bis(2-hydroxyethyl) cocoammonium chloride having 74-75% quaternary material. Adogen 471 is atallow trimethyl ammonium chloride with 49-52% quaternary material.Varisoft 222-90% is a methyl bis(tallow amidoethyl) 2-hydroxyethylammonium methyl sulfate with 89-91% solids. Quaker 2006 is animidazoline type debonder available from Quaker Chemical Co.,Conshohocken, Pa.

                  TABLE IX                                                        ______________________________________                                        Effect of Quaternary Debonder Type with DMDHEU                                Crosslinking Agent                                                                            Disintegration                                                                            Bulk Density,                                              Type   Energy, revs                                                                              cm.sup.3 /g                                       ______________________________________                                        Variquat 638.sup.(1)                                                                     1        125,000     10.5                                          Adogen 471 1        30,000      14.6                                          Varisoft 222-90%                                                                         2        30,000      19.9                                          Varisoft 727                                                                             2        30,000      18.3                                          Quaker 2006                                                                              4        30,000      19.5                                          ______________________________________                                         .sup.(1) This is a modified Type 1 material in that R.sub.2 and R.sub.3       are 2hydroxyethyl or polyoxyethanol.                                     

Representatives of all the general types of quaternary debonders workedwell, although the modified Type 1 material does not seen as effectiveunder the conditions used as the other materials.

EXAMPLE 1

Nonionic and anionic material additives are not substantive to cellulosefibers in an aqueous slurry unless the electrical charge on the fibersurface is made more compatible. This is normally done by adding one ofthe class of papermaking chemicals generally called retention aids priorto the addition of the nonionic or anionic composition. These are mosttypically cationic materials that are substantive to the fibers and makethe surface charge more positive. When anionic or nonionic debonders areused in the present invention they can be added at the wet end, prior tosheeting, or after the sheet is formed. When wet end addition is chosena cationic retention aid is normally required. If a shower over theforming wire or press section, or a pad bath, is used the retention aidis normally not necessary since most of the debonder remains with thewater entrapped in the sheet.

A series of experiments was made to show disintegration energy and bulkvalues with the two modes of addition using cationic, anionic andnonionic debonding agents. For the wet end addition of the nonionic andanionic materials, 0.5% (5 kg/t) of the retention aid Reten 210 wasadded to the fiber slurry prior to the addition of the debonder. Retenis a registered trademark of Hercules, Inc., Wilmington, Del., for avery high molecular weight polyacrylamide having approximately 2-4 mol%cationic sites. No retention aid was used with the cationic material.The cationic debonder was Varisoft 727, described in Example 10; thenonionic material was Triton X-100, described in Example 4; and theanionic was a sodium linear alkyl sulfonate composition with 26.8%active material obtained from Chemithon Corp., Seattle, Wash. These wereall used in dosages of 5 kg/t of the as received material. Those samplesin which the debonder and latent crosslinking agent were added aftersheet formation were prepared according to the procedure of Example 4,with the two materials being mixed in the same treating solution. Allsamples were made using 15 kg/t as received of Arotex 900 crosslinkingagent in the treating bath. Relative dispersibility was evaluated by thecriteria set forth in Example 8, with the exception that here thesamples were retained in the British Disintegrator for a sufficientnumber of revolutions to obtain a relatively smooth slurry. Results wereas follows.

                  TABLE X                                                         ______________________________________                                        Point of Addition of Debonding Agent                                                                       Bulk                                             Debonder                                                                              Point    Disintegration                                                                            Density,                                                                             Relative                                  Class   Addition Energy, revs                                                                              cm.sup.3 /g                                                                          Dispersibility                            ______________________________________                                        Cationic                                                                              Wet End   30,000     17.9   1                                                 Pad Bath  30,000     18.3   1                                         Nonionic                                                                              Wet End  138,000     11.9   3                                                 Pad Bath  62,500     16.9   2                                         Anionic Wet End  175,000     11.0   3                                                 Pad Bath 112,500     13.0   3                                         ______________________________________                                    

Under the conditions of the present test the cationic debonder was themost efficient class of material. Pad bath addition was more efficientfor the nonionic and anionic debonders than wet end addition. This maybe due to an incompatibility or zeta potential unbalance between theparticular type or concentration of retention aid and debonder. It isexpected that with additional experimentation similar results would beobtained for wet end and pat bath addition. The particular anionicsystem chosen for these samples was not particularly efficient.

EXAMPLE 12

Wet tensile strength is believed to be one measure of the ease ofreslurrying a sheeted material. An additional set of samples was made insimilar fashion to those of Example 9. Wet tensile strength was measuredon specimens taken from the Noble and Wood handsheets. Measurements weremade using horizontal specimens 100 mm wide and 80 mm between grips,with a head speed of 1/3 mm/sec. Values were as noted in Table XI.

                  TABLE XI                                                        ______________________________________                                        Wet Tensile Strength Values                                                                     Tensile Strength,                                           Treatment         kN/m                                                        ______________________________________                                        None               8                                                          Crosslinked, no softener                                                                        89                                                          Crosslinked, with softener                                                                      30                                                          ______________________________________                                    

The combination of softener with the crosslinked pulp reduced wettensile strength to 1/3 of that without softener.

EXAMPLE 13

While some latent crosslinking reagents require additional heating atelevated temperatures after the sheet is normally dried, in order toeffect reasonably complete reaction with the cellulose, others willreact sufficiently under normal drying conditions. The use of ureanitrate as a catalyst for the urea-based latent crosslinking materialsgenerally eliminates the need for post-drying heating. This materialappears to be more active than the normally used inorganic salts or saltmixtures. Urea nitrate can be made with equimolar portions of urea andnitric acid under aqueous reaction conditions, using the method ofHebeish and Ibraham, Textile Res Jour., 52 (2):116-122 (1982).

A series of samples was made following the procedure of Example 5.Arotex 900 DMDHEC latent crosslinker was used in pad bath percentagesvarying between 2.5% and 20% with urea nitrate present in the bathequivalent to 3.3% of the DMDHEC, as calculated on a dry materialsbasis. Samples for testing were dried to about 4% moisture contentwithout any additional post drying heating. The sample temperaturesprobably did not exceed about 90° C. at any time. Bulk densities and airresistance values age given in the following table.

                  TABLE XII                                                       ______________________________________                                        Bulk Density and Air Resistance of Low Temperature                            Crosslinked Sheets                                                            As Received                                                                             Crosslinker            Air Resistance                               Crosslinker                                                                             Solids Based                                                                             Handsheet Bulk                                                                            Pressure Drop,                               in Pad Bath, %                                                                          on Pulp, % Density, cm.sup.3 /g                                                                      mm                                           ______________________________________                                        0         0          3.0         47.2                                         2.5       1.0        6.3         17.8                                         5.0       1.9        10.0        13.2                                         10.0      3.8        15.4        3.1                                          15.0      5.7        20.5        1.8                                          20.0      7.6        22.5        1.5                                          ______________________________________                                    

Bulk and air resistance results are generally comparable with thosereported in Table VI where a post drying reaction period of 1.5 minutesat 150° C. was used.

EXAMPLE 14

Urea nitrate was used as in the previous example as the catalyst forArotex 900 DMDHEU latent crosslinker solution. The urea nitrate was madeby dissolving 23.1 g of urea in 462 mL, of water. To this was added 33mL of 70% nitric acid (containing 32.7 g HNO₃). A sufficient time wasallowed for the reaction to go to completion. A solution was then madeof 923 mL of as received Aerotex 900 in 4615 mL water. To this was addedthe above catalyst solution.

A bleached southern pine kraft pulp previously treated with a debondingagent and at about 5% moisture content was treated with the abovesolution on a basis of about 1 mL latent crosslinker solution per gramof pulp. The treated pulp was crosslinked in a drying oven. Samples ofthe untreated pulp, pulp immediately after the application of thecrosslinking agent, and crosslinked pulp were slurried in distilledwater and the pH measured immediately, after 10 minutes and after 25minutes with the following results.

                  TABLE XIII                                                      ______________________________________                                                        pH Value After Mixing                                         Sample            <1 min    10 min  25 min                                    ______________________________________                                        Untreated pulp    4.68      4.85    4.86                                      Treated pulp, wet uncrosslinked                                                                 4.70      4.84    4.90                                      Treated pulp, dried crosslinked                                                                 5.07      5.20    5.30                                      ______________________________________                                    

It is evident that the pH of the system is not reduced below a valuecommonly experienced on a conventional paper machine.

EXAMPLE 16

In a test similar to the above, a solution of Aerotex 900 DMDMEC latentcross linking agent was mixed with Aerotex Accelerator 9 catalystsolution. As was noted earlier. This is a solution of acidic saltsbelieved to be a mixture of aluminum and magnesium chlorides. Two partsof the Aerotex 900 were used with one part Accelerator 9 on a solidsbasis. The latent crosslinking solution was applied as above to adebonder treated bleached southern pine kraft similar to that of theprevious example not from a different mill lot having higher initial pH.As above, pH was measured on the untreated pulp, the wet treated butuncrosslinked pulp, and the dried crosslinked pulp. The results aregiven in the following table.

                  TABLE XIV                                                       ______________________________________                                                        pH Value After Mixing                                         Sample            <1 min    10 min  25 min                                    ______________________________________                                        Untreated pulp    6.40      5.77    6.42                                      Treated pulp, wet uncrosslinked                                                                 4.22      4.25    4.20                                      Treated pulp, dried crosslinked                                                                 4.05      4.30    4.25                                      ______________________________________                                    

As before the pH of the system was above 4 at all times. This is a levelquite compatible with modern papermaking practice.

It will be apparent to those skilled in the art that many departures canbe made from the present description and examples while remaining withinthe spirit of the invention. The invention is to be considered as beinglimited only by the following claims.

We claim:
 1. A method of forming a cellulose product comprising thesteps of:forming an aqueous slurry of unrefined cellulosic fibers;adding to the pulp slurry a cationic debonding agent in an amount ofabout 0.1-20 kg/ton; forming a wet sheet from the pulp slurry; addingfrom about 0.1 to 20 kg/ton, of a water soluble or water dispersiblelatent cellulose crosslinking agent to the wet sheet, wherein thecrosslinking agent is a polycarboxylic acid or is selected from thegroup of urea derivatives consisting of methylolated urea, methylolatedcyclic ureas, methylolated lower alkyl substituted cyclic ureas,dihydroxy cyclic ureas, lower alkyl substituted dihydroxy cyclic ureas,and methylolated dihydroxy cyclic ureas; and mixtures thereof; andimmediately after adding the crosslinking agent, drying the product fora period of time of from about 1 to about 5 minutes at a temperatureabove 100° C. to dry crosslink the cellulose with the crosslinkingagent.
 2. A method of making a crosslinked cellulose product whichcomprises the steps of:providing wet, unrefined cellulose fibers havinga moisture content greater than 30%. providing within said fibers adebonding agent and a water soluble or water dispersible latentcellulose crosslinking agent, wherein the crosslinking agent is apolycarboxylic acid or is selected from the group of urea derivativesconsisting of methylolated urea, methylolated cyclic ureas, methylolatedlower alkyl substituted cyclic ureas, dihydroxy cyclic ureas, loweralkyl substituted dihydroxy cyclic ureas, and methylolated dihydroxycyclic ureas; and mixtures thereof; immediately after providing thecrosslinking agent, drying the fibers at an elevated temperature tosimultaneously dry the fibers and dry crosslink the fibers with thecrosslinking agent.
 3. The method of claim 2 in which the debondingagent is added to an aqueous slurry of cellulosic fibers prior toformation of the cellulose product, and the latent crosslinking agent isadded subsequent to said formation.
 4. The method of claim 2 in whichthe debonding agent and latent crosslinking agent are both addedfollowing said formation.
 5. The method of claim 4 in which thedebonding agent is added prior to the addition of the latentcrosslinking agent.
 6. The method of claim 2 in which the debondingagent is chosen from anionic, nonionic, and cationic debonding materialsand mixtures thereof.
 7. The method of claim 6 in which the debondingmaterial is a cationic material selected from the group ##STR2## whereR₁ and R₂ are methyl, ethyl, or hydroxyethyl, R₃ is a hydrocarbon having1-40 carbon atoms, R4 is a hydrocarbon having 10-40 carbon atoms, E isan oxyalkylene group having 2 or 3 carbon atoms, m is an integer from1-20, n is an integer from 0-20, and X is Cl or SO₄, said hydrocarbonsubstituents being selected from linear and branched alkyl or alkenylgroups, and branched and linear alkyl and alkenyl substituted phenylgroups.
 8. The method of claim 6 in which the debonding agent is anonionic material selected from the group of adduct type reactionproducts of a fatty aliphatic alcohols, fatty alkyl phenols and fattyaromatic and aliphatic acids with ethylene oxide, propylene oxide andmixtures thereof, wherein said fatty portion is a hydrocarbon chainhaving at least 8 carbon atoms; and partial fatty acid esters ofpolyvalent alcohols and anhydrides thereof wherein said alcohol oranhydride has 2-8 carbon atoms.
 9. The method of claim 6 in which thedebonding agent is an anionic material selected from sulfated fats,fatty esters, and fatty alcohols; fatty alkyl substituted aromaticsulfonic acids, and mixtures thereof wherein said fatty substituentgroups have from 8-40 carbon atoms.
 10. The method of claim 8 whichfurther includes adding a cationic retention aid to the cellulose priorto the addition of the nonionic debonding agent.
 11. The method of claim9 which further includes adding a cationic retention aid to thecellulose prior to the addition of the anionic debonding agent.
 12. Themethod of claim 11 which includes adding the latent crosslinking agentto the cellulosic product in the wet press section of a paper machine.13. The method of claim 2 in which the latent crosslinking agent isselected from the group consisting of methylolated urea, methylolatedcyclic ureas, methylolated lower alkyl substituted cyclic ureas,dihydroxy cyclic ureas, lower alkyl substituted dihydroxy cyclic ureas,methylolated dihydroxy cyclic ureas, and mixtures thereof.
 14. Themethod of claim 13 in which the latent crosslinking agent is dimethylolurea.
 15. The method of claim 13 in which the latent crosslinking agentis dimethylol dihydroxyethyleneurea.
 16. The method of claim 13 in whichthe latent crosslinking agent is dihydroxyethyleneurea.
 17. The methodof claim 13 in which the latent crosslinking agent is dimethylolethyleneurea.
 18. The method of claim 13 in which the latent crosslinking agentis 4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone.
 19. The method of claim13 which further includes adding an effective amount of an acidiccatalyst to increase the reaction rate between the cellulose andcrosslinking agent.
 20. The method of claim 20 in which the acidiccatalyst is urea nitrate.
 21. The method of claim 2 in which the latentcrosslinking agent is a polycarboxylic acid.
 22. The method of claim 21which includes a catalyst selected from alkali metal salts ofphosphorous-containing acids.
 23. The method of claim 1 in which thedebonding agent is present in an amount of about 0.1-20 kg/t ofcellulose fiber.
 24. The method of claim 23 in which the debonding agentis present in an amount of about 1-10 kg/t of cellulose fiber.
 25. Themethod of claim 1 in which the crosslinking agent is present in anamount of about 2-200 kg/t of cellulose fiber.
 26. The method of claim25 in which the debonding agent is present in an amount of about 20-200kg/t of cellulose fiber.
 27. The crosslinked cellulosic pulp product ofclaim
 2. 28. The crosslinked cellulosic pulp product of claim
 15. 29.The crosslinked cellulosic pulp product of claim
 18. 30. The crosslinkedcellulosic pulp product of claim
 21. 31. The method of claim 20 in whichthe pH of the treated product remains above about 4 after addition ofthe catalyst.
 32. The method of claim 21 wherein the crosslinked agentis butanecarboxylic acid.
 33. The method of claim 2 further comprisingthe step of adding an effective amount of an acid catalyst to increasethe reaction rate between the cellulose and crosslinking agent.
 34. Themethod of claim 33 wherein the pH of the treated product remains aboveabout 4 after addition of the catalyst.
 35. A method of making a wetformed, crosslinked cellulose product which comprises the stepsof:forming the cellulose product from unrefined cellulose fibers andproviding within said product while the product is at a moisture contentgreater than 30%, a cationic debonding agent and a water soluble orwater dispersible latent cellulose crosslinking agent, wherein thecrosslinking agent is a polycarboxylic acid or is selected from thegroup of urea derivatives consisting of methylolated urea, methylolatedcyclic ureas, methylolated lower alkyl substituted cyclic ureas,dihydroxy cyclic ureas, lower alkyl substituted dihydroxy cyclic ureas,and methylolated dihydroxy cyclic ureas; and mixtures thereof;immediately after providing the crosslinking agent, drying the productat a temperature of at least 100° C. for a period of from about 1 toabout 5 minutes; and simultaneously with drying, dry crosslinking thecellulose with the crosslinking agent in the presence of an effectiveamount of an acid catalyst to increase the reaction rate between thecellulose and crosslinking agent, while maintaining the pH above about4.
 36. The method of claim 35 wherein the debonding agent is a cationicdebonding agent.
 37. The method of claim 35 wherein the crosslinkingagent is a polycarboxylic acid crosslinking agent.
 38. The method ofclaim 35 further comprising the step of reslurrying the sheet aftercrosslinking.